3i6 
Atoms  and  Chemical  Valence. 
Am.  Jour.  Pharm. 
May,  1920. 
Bohr,  Sommerfeld,  and  others  have  developed  an  extensive 
and  very  successful  theory  of  spectra  upon  the  hypothesis  that  the 
electrons  in  atoms  are  in  rapid  rotation  in  plane  orbits  about  the 
nucleus  in  much  the  same  way  as  the  planets  revolve  around  the 
sun.  Stark,  Parson,  and  G.  N.  Lewis,  on  the  other  hand,  starting 
from  chemical  evidence,  have  assumed  that  the  electrons  are  sta- 
tionary in  position.  It  should  be  noted  that  Bohr's  theory  has  had 
its  greatest  success  when  applied  to  atoms  or  ions  containing  only 
one  electron  and  that  it  seems  incapable  of  explaining  the  chemical 
or  ordinar}^  physical  properties  of  even  such  simple  elements  as 
lithium,  carbon,  or  neon. 
The  two  theories  can,  however,  be  reconciled  if  we  consider  that 
the  electrons,  as  a  result  of  forces  which  they  exert  on  one  another, 
rotate  about  certain  definite  positions  in  the  atom  which  are  dis- 
tributed symmetrically  in  three  dimensions.  Thus  for  atoms  con- 
taining only  a  single  electron  the  chemical  theory  is  in  agreement 
with  Bohr's  theory.  But  for  an  atom  such  as  neon  the  eight  elec- 
trons in  the  outside  layer  would  revolve  around  positions  which  are 
located  about  the  nucleus  in  the  same  way  that  the  eight  corners 
of  a  cube  are  arranged  about  the  center  of  the  cube.  This  struc- 
ture is  not  inconsistent  with  those  parts  of  Bohr's  theory  which 
have  received  experimental  confirmation.  In  fact.  Born  and 
Lande,^  starting  with  Bohr's  theory  and  without  knowledge  of 
Lewis'  work,  arrived  at  exactly  this  conception  of  the  structure  of 
atoms  {i.  e.,  the  cubic  atom)  from  a  study  of  the  compressibility 
of  the  salts  of  the  alkali  metals. 
The  atomic  numbers  and  the  properties  of  the  inert  gases  fur- 
nish us  with  a  clue  to  the  arrangement  of  the  electrons  within  atoms. 
The  low  boiling  point,  the  high  ionizing  potential,  the  chemical 
inertness,  etc.,  of  helium  prove  that  the  arrangement  of  the  elec- 
trons in  the  helium  atom  is  more  stable  than  that  in  any  other 
atom.  Since  this  atom  contains  two  electrons  we  must  conclude 
that  a  pair  of  electrons  in  the  presence  of  a  nucleus  represents  a 
very  stable  group.  It  is  reasonable  that  with  elements  of  higher 
atomic  numbers  there  should  be  an  even  greater  tendency  for  this 
stable  pair  of  electrons  to  form  about  the  nucleus.  There  are  two 
sets  of  facts  which  furnish  conclusive  experimental  evidence  that 
this  stable  pair  exists  in  all  atoms  above  helium. 
^  Verh.  d.  phys.  Ges.,  20:  210,  1918. 
